1. Field of the Invention
The present invention relates to printed circuit boards (PCBs), and particularly to a PCB with reduced single-ended impedances of signal traces thereof.
2. Description of Related Art
Referring to FIG. 1, a partial cross-sectional view of a conventional PCB 1 is illustrated. As shown, the PCB 1 includes a differential pair 10 having a positive differential trace D+ and a negative differential trace D− respectively disposed within two signal layers in a broadside-coupling mode, and two ground layers 20 arranged above and below the differential pair 10 respectively. A preferred value of the differential impedance between the positive differential trace D+ and negative differential trace D− is approximately 100 ohms, and a preferred value of the single-ended impedance of either of the positive differential trace D+ and negative differential trace D− is approximately 50-60 ohms.
However, in this broadside-coupling mode of the conventional PCB 1, the single-ended impedance of either of the positive differential trace D+ and negative differential trace D− may be too much, thereby reducing transmission quality thereof. Furthermore, each conventional ground area is formed as an independent layer, which occupies lots of space of the PCB 1.
What is desired, therefore, is to provide a PCB which overcomes the above problems.